In general terms, microphones are devices which convert, or transduce, acoustical energy into electrical energy. It is well known that microphones which use a moving diaphragm are sensitive to vibration caused by shocks or movement, and that when microphones are subjected to forces causing vibration, they produce an unwanted signal. The sensitivity to vibration is a function of design, and thus varies according to the type of microphone.
Receivers (sometimes called "speakers" or alternatively "earphones", according to the context in which the term is used) are devices which transduce electrical energy into acoustical energy. Their construction includes a motor and diaphragm system which is driven by an electrical input signal. When in operation, the motor and diaphragm system produce out-of-balance forces in the form of structure-transmitted vibration.
Appliances such as hearing aids (which use a microphone and receiver simultaneously), small tape recorders (which contain microphones and sometimes receivers) and hand-held transceivers (which alternately use a microphone and a receiver) require their transducers to be protected against unwanted structure-borne forces.
It is known that the sensitivity of these transducers (microphones and receivers) to the reception or transmission of vibration or shock energy is always a maximum when the forces which cause the unwanted movement are applied in a direction normal to the plane of the diaphragm of the transducer. The function of a transducer mounting, therefore, is to locate the transducer within a defined space and to isolate it as much as possible from the reception or transmission of unwanted structure-borne vibrational forces.
The usual transducer mounting system used in hearing aids and other small electro-acoustical appliances comprises a short length of hollow resilient tubing (which transmits acoustical energy to the transducer in the case of a microphone or from the transducer in the case of a receiver) together with at least one buffer which is remote from the acoustic input or output of the transducer. The buffer or buffers are usually loose fitting and act in compression when in contact with the walls enclosing the defined space. The effectiveness of the vibrational isolating properties of this design of suspension depends largely on the compliance and damping properties of the material that is used to manufacture the tubing and the buffers. A high compliance is always sought so that the resonance frequency of the suspension system is both low and away from the operating frequency range of the appliance. A high degree of damping is also sought to minimise direct transmission of energy through the suspension points. These material characteristics are difficult to obtain in practice and always place stringent constraints on manufacturing procedures. Despite this knowledge of the required characteristics of suspension systems, these systems remain a compromise solution in practice.